The invention relates to image processing, and more particularly relates to processing of images produced by scintillation cameras. In its most immediate sense, the invention relates to real-time image processing of nuclear medicine data in such a manner as to make it possible to correct for mispositioning of the patient or for collection of insufficient data before a patient study is concluded.
In a nuclear medicine study, a radioisotope is administered to the patient and the gamma radiation which exits the patient's body is collimated and converted to scintillation flashes in a scintillation crystal. These flashes (or events) are detected by photomultipliers which are placed in operative relation with the crystal and images of the patient's body may be formed using signal processing circuitry that processes electrical signals produced by the photomultipliers.
To produce a two-dimensional (planar) or three-dimensional (tomographic) image of e.g. the patient's heart, it is necessary to collect large quantities of data. This requires time; a typical study may last for 30 minutes or more. At present, the data thus collected is not utilized "on the fly"; only after the study has been completed by acquiring a predetermined number of events is it possible to determine whether the study has in fact elicited the information of interest.
For example, let it be assumed that an equilibrium gated blood pool study is to be conducted in order to determine the ejection fraction of the patient's left ventricle. To do this, the camera must be so positioned with respect to the patient that the blood pool in the left ventricle does not overlap the blood pools in the other three chambers of the patient's heart.
It is difficult if not impossible to know on an a priori basis whether the positioning between camera and patient is exactly correct, given that the orientation of the heart within the patient's body can vary over a comparatively wide range. Thus, it often happens that at the end of a study, the radiologist discovers that such positioning is slightly off. This means that the radiologist must choose between conducting another study (and thereby exposing the patient to more radiation and tying up the camera for an additional period of time) or using a suboptimal study.
Furthermore, in such a study, the proper measure to use in determining when the study should end is the number of events originating at the patient's left ventricle and not the total number of events which are detected by the camera. This is because the distribution of the radioisotope within the patent's body may be otherwise than anticipated and also because events from the heart region of interest may be attenuated differently than expected. In conventional scintillation cameras, the software does not permit incoming events to be examined "on the fly", so that it is impossible to distinguish between events which originate in the left ventricular blood pool and those which originate elsewhere. This makes it necessary to use total events to determine study duration, even though this is not the proper measure to be used.
It would therefore be advantageous to provide a method and apparatus which would permit incoming image data to be evaluated "on the fly" so that, e.g., any mispositioning of the patient or camera could be corrected immediately and the study restarted without a prolonged wait and so that only certain data (e.g. image data from a particular region of interest) would be used to measure parameters of interest (e.g. study duration).
One object of the invention is to provide method and apparatus which would permit a radiologist or radiological technician to determine, at an early stage of a patient study, whether the data collected during the study appears to meet the requirements applicable to the study.
Another object is to provide such method and apparatus which would permit positional corrections of a patient and/or a scintillation camera to be made at an early stage of a patient study so as to avoid completion of the study under inappropriate conditions.
Still another object is, in general, to improve on known methods and apparatus used in nuclear medicine.
The invention proceeds from the realization that a combination of two known (and previously independently used) approaches in principal component analysis (PCA) and factor analysis (FA), together with a novel definition of noise, permits incoming data of the type which is generated by a scintillation camera to be analyzed in real time.
The first of these approaches is exemplified by the work of D. C. Barber, of Sheffield University in the United Kingdom. Barber's work is time-based PCA and FA. In the Barber methodology, a series of images are acquired, one after the other, and recorded. Because the quantity of data thereby acquired is considered unmanageably large, Barber combines (as by a sort of averaging) pixels in the individual frames to construct macro-pixels. Then, Barber retrospectively analyzes the macro-pixels to see if they can be grouped into categories which are temporally covarient. Where this is possible, the results of the Barber PCA and FA is to produce correlated curves showing time-based activity of, e.g., a patient's brain or kidney.
The second of these approaches is exemplified by the work of M. Samal, of Charles University in Czechoslovakia. In the Samal methodology, a series of images is likewise acquired and recorded, but the retrospective analysis is carried out with unreduced data to see if they can be grouped into categories which are area related. Where this is possible, the results of the Samal PCA and FA is to produce correlated curves showing area-based activity of, e.g., two moving phantoms which overlap and expose each other as they move.
In accordance with the invention, each study of interest is analyzed in advance to identify a discrete number of categories of image data which may reasonably be expected to be covarient both regionally and temporally. For example, if the study is a cardiac study, it may reasonably be supposed that there will be a strong linkage between the time variation of all pixels associated with the patient's atria and a second strong linkage between the time variation of all pixels associated with the patient's ventriculi. Thus, it may be supposed that the overall image of the heart during a cardiac study may be approximated to the first order by a combination of two time-varying subimages, one of which subimages represents an atrial image and the other of which subimages represents a ventricular image. Further, the image presented to the camera will be a superposition of these two images as viewed from a particular angle of rotation.
In further accordance with the invention, image data which does not fit into the above-established categories is defined to be noise and is ignored. Thus, in accordance with the invention, there is an a priori definition, on a study-by-study basis, of the sort of image data which contains diagnostic information and the sort of image data which does not; the first sort is collected into categories and used for analysis and image production and the second sort is discarded. Because Poisson noise is a major factor in, e.g., nuclear medicine image data output from the detector head, a discarding of noncategorizable information effects a major reduction in the amount of data which must be processed in subsequent steps and thereby drastically improves the speed of image processing without any degradation of image quality.
In further accordance with the invention, incoming data is examined and binned in the above-referenced pre-established categories. The binned data may then be tested at intervals to make sure that predetermined criteria are met. For example, incoming data collected during a nuclear medicine cardiac study and binned into "atrial" and "ventricular" categories will reflect the orientation between the heart and the camera. Thus, even if there is insufficient information available to form diagnostic images, the statistics of the incoming information will permit the technician to determine that, e.g., a mispositioning of the patient with respect to the camera has taken place and should be corrected.